The review looked at children and adults with a range of chronic conditions including diabetes, arthritis, eating disorders, asthma, cancer and Alzheimer's disease.

After looking at 28 trials involving more than 4000 patients, it found that using the internet improved knowledge and gave patients a sense of social support.

But web-based programs had no effect on behaviour and didn't encourage people to change their lifestyle. They also had a negative effect on long-term health.

"Consumers who wish to increase their knowledge or social support amongst people with a similar problem may find [using the internet] helpful," the review said.

"However, consumers whose primary aim is to achieve optimal clinical outcomes should not use [it]."

Should we abandon the net?

Australian researcher Professor Richard Wootton, director of research at the Centre for Online Health at the University of Queensland, said people shouldn't rush to the conclusion that turning to the internet in response to illness was a health hazard.

He said researchers in Queensland and Canberra had shown the internet was valuable in delivering cognitive behaviour therapy for conditions like non-clinical depression and obsessive-compulsive disorder, areas excluded from the Cochrane review.

"I think it's a very interesting finding in a new and potentially important field but it's hard to interpret what it means at this rather early stage of the game," he said.

"It may be that when you get larger studies and more of them this [negative effect] may disappear."

Lead researcher Dr Elizabeth Murray of UCL described the finding as "unexpected and provocative".

Murray said more research was needed to establish whether the negative health outcomes were because patients became over-assured, thereby losing the motivation to change bad habits, or made a decision to prioritise short-term rather than long-term benefits.

Knowing about something and changing behaviour were also two different things, as smoking attested, she said.

The reviewers said the study had important policy implications because of the worldwide proliferation of online health programs, often at significant public expense.

*EDITOR'S NOTE: The Cochrane Collaboration has since retracted the review after being alerted to methodological errors.